A comparative study is made of the three groups (Cyphophthalmi, Laniatores and Palpatores) into which Opilionida are currently subdivided. Methods, principles and terminology, adopted in this paper, correspond with those of parts I and II of the present series of studies in Chelicerata.Special attention is paid to mouthparts and appendages, to opilionid relationships, and to the evolution of chelicerate characters. It is demonstrated that Cyphophthalmi present all the essential opilionid characters, and evidently do not constitute a separate group. It is also demonstrated that Opilionida are not closely related to Actinotrichid or Anactinotrichid mites, but to other groups with a so-called myliosoma (mouthparts associated with coxisternal feeding); these groups constitute the new superclass Myliosomata. The phylogenetic relationships of Epimerata (Palpigradi and Actinotrichid mites) and Cryptognomae (Anactinotrichid mites and Ricinulei) are discussed, and these groups are regarded as representatives of a new superclass Acaromorpha. As a result of these conclusions, a new diagram is prepared of the phylogenetic relationships of all extant groups of Chelicerata.